The present invention relates generally to a method of producing a microstructure bearing composite article that exhibits superior scratch resistance qualities. More particularly, the present invention relates to a method of increasing scratch resistance of a brightness enhancement film or an optical lighting film.
The geometrical features and use of microstructure bearing articles as a brightness enhancement film are shown in Whitehead, U.S. Pat. No. 4,542,449. Other examples of microstructure bearing articles, which are useful as brightness enhancement films, include Lu et al., U.S. Pat. No. 5,175,030, and Lu, U.S. Pat. No. 5,183,597. These patents disclose microstructure bearing composite articles and a method of molding the microstnicture bearing composite articles. The Lu et al. patent and the Lu patent address forming the microstructure so as to exhibit desired optical properties, such as total internal reflection.
Microstructure bearing articles are made in a variety of forms. One such form includes a series of alternating tips and grooves. One example of such a form is brightness enhancement film, which has a regular repeating pattern of symmetrical tips and grooves. Other examples include patterns in which the tips and grooves are not symmetrical and in which the size, orientation, or distance between the tips and grooves is not uniform. When the included angle of the grooves is in the range of about 70.degree. to about 120.degree. and especially in the range of about 80.degree. to about 100.degree., the article with alternating tips and grooves is useful as a brightness enhancement film.
One major drawback of prior art radiation-cured brightness enhancement films and optical lighting films is that the tips of the microstructure are susceptible to mechanical damage. For example, light scraping with a fingernail or other hard, relatively sharp edge can cause the tips of the microstructure to break. Conditions sufficient to break the tips of prior art microstructures are typically experienced during normal handling of brightness enhancement films in the manufacturing of liquid crystal displays for lap-top computers.
When microstructure peaks are broken, the reflective properties of the affected peaks are reduced and the transmitted light scattered to virtually all forward angles. Hence, when the brightness enhancement film is in a display, and the display is viewed straight on, scratches in the brightness enhancement film are less bright than the surrounding, undamaged area of the film. However, when the display is viewed at an angle near or greater than the "cutoff" angle, the angle at which the image on the display is no longer viewable, the scratches look substantially brighter than the surrounding, undamaged area of the film. In both situations, the scratches are very objectionable from a cosmetic standpoint, and brightness enhancement film with more than a very few, minor scratches is unacceptable for use in a liquid crystal display.
Prior art brightness enhancement films include "Brightness Enhancement Film", a version made from polycarbonate thermoplastic, sold by Minnesota Mining and Manufacturing Company, St. Paul, Minn. and "DIAART" a version made of a radiation cured microstructured layer on a polyester base, sold by Mitsubishi Rayon, Tokyo, Japan. Both of these versions are not as scratch resistant as desired.